I just came across a discussion on this matter on a foreign forum:
Snap is really the product of a commercial company more & more linked with Microsoft : Canonical. (Windows snaps are planned btw)
They have rights & licence over the snaps and include tracking as a commercial argument for devs.
Flatpak il much more a community, Linux centered project:
Without going to the commercial / Microsoft link topic, one difference that does strike between Snaps and Flatpaks is the control over the distribution from Canonical and Flatpak Team respectively.
Snap Store and Flathub seem the de facto main distribution sources for Snaps and Flatpaks. Although they can both be distributed through other sources, i read Canonical keeps parts of the Snap Store closed source, effectively making it harder reproduce than the fully open-source Flathub.